Simplify seven square root of 99

To simplify \( 7\sqrt{99} \), we start by breaking down the square root:

1. Factor \( 99 \) into its prime factors:
\[
99 = 9 \times 11 = 3^2 \times 11
\]

2. Now, we can express the square root:
\[
\sqrt{99} = \sqrt{9 \times 11} = \sqrt{9} \times \sqrt{11} = 3\sqrt{11}
\]

3. Substitute back into the expression:
\[
7\sqrt{99} = 7 \times 3\sqrt{11} = 21\sqrt{11}
\]

Thus, the simplified form of \( 7\sqrt{99} \) is:
\[
\boxed{21\sqrt{11}}
\]